Paul, I am not familiar with that model, I am more familiar with 5's and A 5's, so take my comments with a grain of salt. While the overall shape looks like a 15, the lever cap, and the lever cap screw don't look the same as in this example,
early 15
I have some non adjuster Norris's and the norris stamp is similar to the one in the link. Here are a variety of Norris stamps from 5's , and I don't see anything similar in the photo that you posted.
Norris stamps on lever cap
Also, it is hard to tell from the few photo's you posted, but the lever caps in the 5's appear to have more curve in the neck than the photo you posted.
As Andy says, Norris sometimes sold their planes unmarked to other sellers, like Buck. Also, the plane is likely over 100 years old, so who knows whether some of the things that look odd to me are replacements to the original. Finally, as some have suggested, it is hard for us really have firm understanding of how things were made back then. While there are some things that would have been standardized, the actual end product may have looked different on different days depending on the workman, and conditions, of the shop on the day it was produced and even what the customer asked for.
